1. Field of the Invention
The present invention pertains to measurement of fluid flow. More particularly, the present invention relates to electromagnetic flowmeters of the Faraday effect type. Such apparatus may be used to measure flow of any fluid which is an electrical conductor, or which can be ionized.
2. Description of Prior Art
The concept of employing magnetic induction to measure flow of a conducting fluid is well documented. U.S. Pat. No. 2,149,847 to Kolin discloses basic details of a system for such measurement. Essentially, the fluid flow is constrained to pass along a conduit through a magnetic field, with the conduit oriented at a non-zero angle to the field. A potential gradient is induced across the fluid flow within the magnetic field, proportional to the average fluid velocity, and may be detected through electrodes placed in electrical contact with the flow and disposed on opposite sides of the conduit. Preferably, the magnetic field direction, the direction of fluid flow through the magnetic field, and the alignment of the electrodes are all mutually orthogonal.
Electromagnetic flowmeters find significant application in the medical-biological field. Examples of such application include monitoring blood flow during surgery, and during hemodialysis therapy for patients with chronic renal failure. A variety of flow probes have been designed for such purposes, including catheter probes for insertion within a blood vessel, non-cannulated probes which utilize the blood vessel itself as a conduit, and cannulated probes which include a lumen unit, and measure extracorporeal flow.
Sterilization of that part of the flow probe that contacts the patient may be required. Consequently, it is advantageous to be able to separate the magnet portion of the probe from the electrode structure, and possibly to isolate the former from the need for sterile contact with the patient. The concept of separable magnet and electrode elements is known. U.S. Pat. No. 3,757,773 discloses a catheter probe with a magnet unit that remains external to the blood vessel. Additionally, U.S. Pat. No. 3,659,591 discloses a system wherein electrodes are appropriately placed on the skin of the subject, and a coil, remote from the skin, is used to generate the magnetic field. In Proceedings of the I.R.E. 1959, 1901-1912, H. W. Shirer et al disclosed a separable magnet and lumen type structure. However, it will be appreciated that the location and orientation of the flow conduit, whether a blood vessel or an extracorporeal lumen, as well as placement of the electrodes, relative to the magnetic field affect the potential difference measured for a given flow rate. Thus, to avoid the requirement of calibrating the flow meter for each application, there must be consistency of alignment of the conduit and electrodes relative to the field whenever the probe is reconstructed for a measurement.
Sterilization is a particularly acute problem in the case of extracorporeal probes because the lumen itself must be sterilized. For this reason, a disposable lumen unit, used in conjunction with a non-disposable magnet portion, would yield an economic advantage. The magnet portion may be the more expensive part of the probe, and the cost of sterilization of the lumen could be saved. However, to be as practical as possible, such a probe requires a magnet portion that can be used with a great number of different lumen units without the need for recalibration when lumen units are interchanged. Furthermore, an additional economic advantage could be gained if magnet portions could be made to generate like magnetic fields, so that any lumen unit may be used with any magnet portion without further calibration.
Electromagnetic flowmeters usually employ some form of alternating current signal to generate a magnetic field. This is done to avoid problems associated with d.c. magnetic fields, including the difficulties in amplifying the induced d.c. flow signal, and possible plating of the electrodes. Also, where blood flow is being measured, the resulting d.c. field within the blood can lead to breakdown of the blood constituents. However, an alternating magnetic field causes transformer effects, resulting in a quadrature component signal in the electrodes which may be many times larger than the fluid flow signal. Various measures have been proposed to negate, or at least cope with, the quadrature signal. Some of these are discussed, for example, in U.S. Pat. Nos. 2,808,723; 3,316,762; 3,323,364; 3,329,018; and 3,757,773.
The flow signal is further masked by effects of eddy currents generated in the fluid moving in the magnetic field, with possible attendant polarization of the electrodes, and by capacitive imbalance between the electrode leads. The former problem is also addressed by the aforementioned U.S. Pat. No. 3,329,018.